1. Field of the Invention
The present invention relates to an image forming apparatus, particularly a color tandem image forming apparatus utilizing electrophotography such as a copying machine, a printer, a facsimile and a complex machine of those, more specifically, relates to an image forming apparatus having a hybrid developing apparatus which develops a latent image to which an isolated charged toner is caused to jump from a developing roll holding the isolated charged toner on it using two components developer material, in which non-magnetic toner is charged by means of magnetic carrier.
2. Description of the Related Art
Heretofore, there are kinds of developing methods in an image forming apparatus utilizing electrophotography such as a copying machine, a printer, a facsimile and a complex machine of those. One is a two-components developer material method which uses toner and magnetic carrier; another is one-component developer material method which uses nonconductive toner or conductive toner. Another one is a hybrid developing method which develops a latent image twohich an isolated charged toner is caused to jump from a developing roll holding the isolated charged toner on it using two components developer material, in which non-magnetic toner is charged by means of magnetic carrier.
A two parts developer material method has advantages of such as excellent charging property of toner by carrier, long life of toner as well as uniformity of solid image. On the other hand, it has drawbacks of such as a large and complicated developing apparatus, generation of toner scattering or carrier accompanying as well as varying image quality depending on durability of carrier. A one-component developer material method has advantages of a compact developing apparatus as well as good dot reproducibility while it has drawbacks of the poor durability of a developing roll and a charge roller, which results in selective developing or high maintenance cost of supplies and material for developing apparatus. A hybrid developing method is a high rate image forming method which has good dot reproducibility and a long life, however it has heretofore such problems as generation of developing ghost or toner scattering.
Though a hybrid developing method has been developed as one of measures for a one-component non-contact developing, it has been recently developed as a high rate developing method for a developing method of superimposing multi-colors on one drum, in which a plurality of color images are sequentially formed on one latent image bearing body (a photosensitive body). Attention has been paid to this method as an art of quality color images because a color image with scarce color drift can be obtained by accurately overlapping different colored layers of toner on a latent image bearing body.
However, since developing apparatuses of a number of colors used should be disposed around the latent image bearing body in a developing method of superimposing multi-colors on one drum, the latent image bearing body becomes large in size so as to prevent from designing a compact image forming apparatus. Hence, attention has been paid to a tandem method in which a plurality of electro-photographic process units corresponding to colored toners used are aligned and colored images are formed synchronized with transferring of a transfer member so as to overlapping the colored images on the transfer member. However, this method has a drawback that the image forming apparatus becomes large because electro-photographic process units of kinds of colors should be aligned, though it has an advantage in having a rapid rate of printing. In order to improve measures against this problem, a compact tandem image forming apparatus has been proposed, which is provided with a small-sized image forming unit by narrowing a distance between latent image bearing bodies.
The compact tandem image forming apparatus thus designed has an advantage when it has a vertical developing apparatus in order to minimize a size in width direction of the image forming unit. That is, it is desirable regarding parts layout to dispose a developing apparatus in the upper part of the latent image bearing body. However, in case the developing apparatus is disposed in a vertical manner in a conventional two-components developer material method, flow of developer material, that is, the supply of developer material from a developer mixer to the part adjacent to a latent image bearing body is complicated so that down-sizing of the apparatus is limited, carrier adheres inevitably to the latent image bearing body and toner scatters unavoidably.
Though, as another method, a one-component developer material method in which carrier is not used is proposed, it has a drawback of enhancing color drift, which is the weak point of a tandem type, on account of the torque fluctuation of a latent image bearing body caused by contacting a developing roll to the latent image bearing body. Meanwhile, with a method in which a latent image bearing body is not touched, toner is charged by a charge roll and the toner thickness on the developing roll is restricted by an elastic restricting blade. Thus, additive for toner adheres to the charge roll so that the charging ability is decreased or toner adheres to the restricting blade so that toner thickness becomes uneven, which results in image defects.
Hence, as a measure for resolving the problems, the aforementioned hybrid developing method has attracted attention. Though the hybrid developing method has the problems with regard to generation of ghost and scattering of toner, the method has little torque fluctuation owing to non-contacting between the developing roll and the image bearing body as well as excellent dot reproducibility so as to be able to provide a high-speed image forming apparatus capable of a long life.
A prior art regarding a hybrid developing method is disclosed in U.S. Pat. No. 3,866,574 which proposes a method wherein a non-magnetic toner thin layer is formed on a donor roll (developing roll) disposed untouchably toward a latent image bearing body and the toner is caused to jump to the latent image on the latent image bearing body by an alternative current electric field. While, U.S. Pat. No. 3,929,098 shows a developing apparatus which forms a toner layer by transferring the toner onto a donor roll using a magnetic roll.
However, although these arts enable toner to form a thin layer on a donor roll by means of two-components developer material, a strong alternative current electric field is necessary to separate toner from the donor when the electrostatic charge of the toner is high. The strong alternative current electric field disturbs the toner layer so that a problem concerning superimposing multi-colors arises. In this regard, so called a powder cloud developing method is disclosed in Japanese laid open patent publication, No. JP1992-113474 wherein auxiliary electrodes consisting of wires are provided between a donor roll and a latent image bearing body applying a weak alternative current electric field so as not to disturb the toner developed.
Theoretical aspect of the formation of toner layer on a developing roll using two-component developer material was reported by Toshiba Corporation in the Electrophotography Society Journal 19, 2 (1981) and the art is disclosed in Japanese laid-open patent publication JP1984-121077.
The aforementioned prior arts have a tendency of decreasing image consistency owing to selective developing as coarse particles of the high developing natured toner are apt to selectively jump to the latent image body and fine particles of the highly charged toner remains on the developing roll sleeve. Further, the control of charging toner is complicated so that a high surface potential and a big developing electrical field need to be applied. In this regard, if both toner consumption region and non-consumption region are generated on the developing roll, toner adhering state and toner potential vary on the surface of the developing roll. Thus, as shown in FIG. 3, so called a hysteresis, a phenomenon in which a part of previous developing image appears as a ghost is liable to generate. In FIG. 3, 35 is a solid image consisting of a rectangular black image and 36, 37 are half tone image succeeding and broader than the solid image. When both toner consumption region and non-consumption region are generated on the developing roll, printing the half tone images 36, 37 after the solid image 35 generates the ghost shown in FIG. 3(b). Further in case high consistent images are printed repeatedly, unevenness of image such as mottle in image consistency is liable to occur so that a problem in down sizing the apparatus arises.
In order to overcome this problem, Japanese laid-open patent publication JP1999-231652 discloses a member for scraping the residual toner on a developing roll and a recovery device for the scraped toner, while Japanese laid-open patent publication JP2000-81788 discloses an apparatus using an exclusive recovery roll for assuring recovery of toner on the developing roll. However these methods are necessary for complex structures enabling to make practical for a compact electrophotographic apparatus. Further, Japanese laid-open patent publication JP1995-128983 discloses an apparatus wherein toner on a developing roll is recovered by setting a broad half value region of magnetic density of a magnetic roll as a measure for the hysteresis when using magnetic brushes. Japanese laid-open patent publication JP1988-249164 discloses a control method for a developing apparatus of tandem type wherein degradation of developer material is prevented by bringing down the operation of developing apparatus in an image forming part except for the image forming part whereat a transfer process is performed. In order to prevent big effect for image quality caused by fluctuation of a toner concentration in two-component developer material, Japanese laid-open patent publication JP1998-31366 discloses a developing apparatus of non-contact method for two-component developer material wherein toner consumption is predicted by image data by providing a plurality sets of developing magnetic pole and a trimming magnetic pole for fixing toner replenishing amount on a magnetic roll while a position of the trimming magnetic pole is adjusted so as to be capable of supplying a most appropriate toner amount from the actual toner amount on a latent image bearing body and the predicted toner consuming amount by providing a device for detecting a toner amount on the latent image bearing body.
In a hybrid developing method, decreasing of image consistency owing to selective developing or developing defect, image degradation, developing ghost, toner scattering and sleeve adhesion owing to leaving for a long lime while holding toner on a developing roll are generated. In order to solve the aforementioned problems, Japanese laid-open patent publication JP1994-67546, JP1995-72733 and JP1995-92804 disclose an image forming apparatus having a magnetic roll forming magnetic brushes by two-components developer material, a donor roll (developing roll) holding a toner thin layer supplied by the magnetic roll and electrodes disposed between the donor roll and a latent image bearing body wherein bias consisting of alternate current and direct current voltage are applied to the electrodes, direct current bias is applied to the developing roll and polarity reversible direct current bias by a switch (JP1995-72733) or alternate current bias superposed on direct current (JP1994-67546 and JP1995-92804) is applied to the magnetic roll.
In a developing apparatus described in JP1994-67546 or JP1995-72733, a toner thin layer is formed on the developing roll with magnetic brushes generated on the magnetic roll with direct current potential difference (JP1995-72733) or alternate current bias (JP1994-67546 and JP1995-92804) between the magnetic roll and the developing roll and further a latent image on the latent image bearing body is developed by forming toner cloud near the electrodes with an alternate current bias superposed on direct current applied between the developing roll and the electrodes while the toner on the developing roll is recovered by applying a direct current bias in the direction of peeling toner from the developing roll to the magnetic roll with switching after formation of images (JP1995-72733) and JP1995-92804 or at a definite interval (JP1994-67546) and by applying a direct current bias (JP1995-72733) in the direction of transferring toner on the magnetic roll to the developing roll with switching at the successive image formation, whereby formation of images are prepared to solve the above mentioned problems.
Japanese laid-open patent publication JP1995-72733, U.S. Pat. No. 5,420,375, disclose a hybrid developing apparatus having a magnetic roll forming magnetic brushes by two-components developer material, a donor roll (developing roll) holding a toner thin layer supplied by the magnetic roll and electrodes disposed between the donor roll and a latent image bearing body wherein a bias consisting of direct current and alternate current is applied to the electrodes and to the magnetic roll is applied direct current bias capable of changing polarity with a switch whereby such problems as decreasing of image consistency owing to selective developing or developing defect, image degradation, developing ghost, toner scattering and sleeve adhesion owing to leaving for a long time while holding toner on a developing roll are solved.
That is to say, in the developing apparatus disclosed in JP1995-72733, a toner thin layer is formed on a magnetic roll with a potential difference between the magnetic roll and a developing roll and a latent image on the latent image bearing body is developed by forming toner cloud near the electrodes with an alternate current bias superposed on direct current applied between the developing roll and the electrodes while the toner on the developing roll is recovered by applying a direct current bias in the direction of peeling toner from the developing roll to the magnetic roll with switching after formation of images and by applying a direct current bias in the direction of transferring toner on the magnetic roll to the developing roll with switching at the successive image formation, whereby formation of images are prepared to solve the above mentioned problems.
In JP2000-250294, it is argued that a method in which electrodes are provided between a developing roll and a latent image bearing body disclosed in JP1995-72733 has such draw back as an uneven phenomenon owing to vibration of wires tensed with electrical bias and generation of scars on the developing roll by dust attached instantaneously on the electrodes. A hybrid developing apparatus having electrodes buried in a developing roll is introduced as a prior art. Even in the apparatus using a developing roll with electrodes buried in the roll, carrier attached on the developing roll sticks to images and toner is not effectively supplied to the latent image bearing body because electrodes buried in the roll have a certain distance between them. Consequently, the apparatus generates a phenomenon of empty images when images of high image ratio are continuously printed or a phenomenon of low quality image or low image consistency caused by selective development on account of alternate current bias applied to the magnetic roll and the developing roll so that the patent publication disclosed a hybrid developing apparatus preventing these phenomenon.
Namely, in the patent publication JP2000-250294, a dielectric layer into which electrodes are buried with a small distance is provided on a electroconductive sleeve to constitute a developing roll whereby improving the effectiveness of supplying toner to a latent image bearing body, preventing short-circuiting between electrodes and protecting the surface. In addition, an electrode-relaxation-allowing layer having a dielectric constant with which cumulated electric charges can be diffused and capable of fringe charge penetrating through the coating for a shorter time than several seconds is provided. A brush electrode supplying a bias voltage consisting of alternate current voltage and dielectric current voltage to the electrodes buried in the developing roll is provided between the developing roll and the latent image bearing body and another brush electrode supplying another bias voltage consisting of alternate current voltage and dielectric current voltage to the electrodes buried in the developing roll is provided between the developing roll and magnetic roll, so that, a latent image on the latent image bearing body is developed with a cloud of toner between the latent image bearing body and the developing roll, and toner is reciprocated between the developing roll and the magnetic roll. The alternate current voltage source is common to the both biases and its wave is a square wave, a duty ratio of which is set in such a manner that a time for transferring toner from the magnetic roll to developing roll is shorter than a time for recovering toner from developing roll to the magnetic roll so as to prevent selective transfer of toner and carrier sticking to the developing roll utilizing the difference of inertia between toner and carrier.
The powder cloud developing method disclosed in the Japanese laid open patent publication, No. JP1992-113474 is not commonly practiced because weirs of auxiliary electrodes are apt to become dirty and image degradation generates owing to vibration. Further, the apparatuses disclosed in JP1999-231652, JP2000-81788 and JP1995-128983 have following drawbacks; namely, one needs a scratching device for toner and a recovery roll; another has a cause of degradation of toner durability owing to increasing toner stress by applying a special bias for recovery; yet another needs a long time for forming the layer on the developing roll at the time of developing reducing a developing speed. Further, after processing for a long time, charging property of toner varies greatly on the developing roll owing to degradation of durability of carrier leading to broadening the charging distribution of replenishing toner or recovered toner, which results in toner scattering or image fogging caused by poor toner charge. In addition, as the art needs a troublesome exchange of carrier, it does not have turned to a practical use.
In a tandem type developing apparatus disclosed in JP1988-249164, the action of developing apparatus except for an image forming part processing a transfer process needs to be stopped or the apparatus needs an apparatus or an control device for changing a high voltage applied between the developing roll and the magnetic roll with a high frequency resulting in cost up, while the apparatus has a structure of disposing donor roll, a magnetic roll and a stirrer member in a horizontal way so that downsizing of the apparatus is difficult. In a developing apparatus disclosed in JP1998-31366, a non-contact, two-component developer material method is carried out, while a plurality sets of developing magnetic poles on the magnetic roll and a trimming magnetic pole controlling a toner replenishing amount or a complex structure capable of fixing rotatably at a plurality of positions according to a replenishing amount of toner need to be provided inevitably leading to a complicated control method and a high cost developing apparatus.
An apparatus disclosed in JP1994-67546, JP1995-72733 or JP1995-92804 is operated by the powder cloud method in which the wires of the auxiliary electrodes are likely to get dirty and image degradation caused by vibration occurs. The apparatus disclosed in JP2000-250294 needs to bury electrodes into the developing roll and has a complicated and costly structure as necessary for a brush electrode for supplying the electrodes a superposed bias of AC and DC. The electrodes disposed intermittently in the circumferential direction while toner can not be controlled when the brush electrode becomes unable to contact the electrodes on the developing roll owing to sticking of toner caused by vibration or getting dirty for some reasons.
Further, as mentioned above, when high consistency images are printed successively, fine particles of toner and contamination of toner constituents stick to the developing roll that causes toner filming leading to unevenness of images such as generation of heterogeneous image consistency. These are problems when downsizing the developing apparatus.